Swift:罕见的在线崩溃，只显示someDispatchQueue.sync {

Question

在6个测试人员中，有一个用户在只显示以下内容的线路上遇到崩溃

someDispatchQueue.sync { //Thread 0 crash

someDispatchQueue不能为nil，因为它正被声明为

在课程开始时使用let someDispatchQueue = DispatchQueue(label: "SomeDispatchQueue")。

我能想到的坠机原因只有两个：

A)关于它的调用方式的问题，例如

DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + delay) {
//a few functions in between
    someDispatchQueue.sync {

也许你不能在main.async{}中调用customQueue.sync{}？然而，这个测试人员是唯一经历过这种崩溃的人，而其他5个测试人员也可以99%确定地运行这个功能-没有任何问题。它在我的设备上运行也没有问题。

B)日志的指示行为off

Swift崩溃日志是否应该显示崩溃的确切位置？或者可以少几行？日志中显示以下内容：

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000001, 0x0000000100f2f95c
Termination Signal: Trace/BPT trap: 5
Termination Reason: Namespace SIGNAL, Code 0x5
Terminating Process: exc handler [6599]
Triggered by Thread:  0

Thread 0 name:
Thread 0 Crashed:
0   AppName                         0x0000000100f2f95c closure #1 in FileDB.prepareFile() + 508 (<compiler-generated>:0)
1   AppName                         0x0000000100f39734 partial apply for thunk for @callee_guaranteed () -> () + 20 (<compiler-generated>:0)
2   AppName                         0x0000000100f2ccf4 thunk for @escaping @callee_guaranteed () -> () + 20 (<compiler-generated>:0)
3   libdispatch.dylib               0x000000019770c33c _dispatch_client_callout + 20 (object.m:495)
4   libdispatch.dylib               0x00000001977191f4 _dispatch_lane_barrier_sync_invoke_and_complete + 60 (queue.c:996)
5   AppName                         0x0000000100f2f6b4 FileDB.prepareFile() + 264 (FileDB.swift:138)                                            //line 138 is the someDispatchQueue.sync {
6   AppName                         0x0000000100f79844 ICEFrameworkHandler.initiateWrite() + 728 (ICEFrameworkHandler.swift:254)
7   AppName                         0x0000000100f78e18 closure #1 in ICEFrameworkHandler.delayRun(delay:stamp:) + 140 (ICEFrameworkHandler.swift:164)     //the function that calls the DispatchQueue.main.asyncAfter
8   AppName                         0x0000000100f7e508 thunk for @escaping @callee_guaranteed () -> () + 28 (<compiler-generated>:0)
9   libdispatch.dylib               0x000000019770c33c _dispatch_client_callout + 20 (object.m:495)
10  libdispatch.dylib               0x000000019770eaf8 _dispatch_continuation_pop + 408 (inline_internal.h:2484)
11  libdispatch.dylib               0x000000019771f624 _dispatch_source_invoke + 1224 (source.c:568)
12  libdispatch.dylib               0x00000001977184f0 _dispatch_main_queue_callback_4CF + 560 (inline_internal.h:2525)
13  CoreFoundation                  0x00000001979e76b0 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16 (CFRunLoop.c:1749)
14  CoreFoundation                  0x00000001979e22c8 __CFRunLoopRun + 1708 (CFRunLoop.c:3069)
15  CoreFoundation                  0x00000001979e18f4 CFRunLoopRunSpecific + 480 (CFRunLoop.c:3192)
16  GraphicsServices                0x00000001a1df8604 GSEventRunModal + 164 (GSEvent.c:2246)
17  UIKitCore                       0x000000019bbb5358 UIApplicationMain + 1944 (UIApplication.m:4823)
18  AppName                         0x0000000100f150b4 main + 68 (AppDelegate.swift:19)
19  libdyld.dylib                   0x000000019785d2dc start + 4

@matt

let someDispatchQueue = DispatchQueue(label: "SomeDispatchQueue")

DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {

    someDispatchQueue.sync {
        print("someDispatchQueue.sync") //prints first, no exception
    }
    print("end of asyncAfter()")        //prints second, reaches statement without exception
}

Answer 1

这一行：

DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + delay) {

...proves，当我们到达这里时，我们就在主队列上。好，现在：

    someDispatchQueue.sync {

哇哦！当你在主队列中时，千万不要说sync。这是因为sync的意思是“阻止我；我想停下来等待另一个队列完成下一个任务。”但是“me”是主队列；你永远不能阻塞主队列。